Metal-on-metal (MOM) prosthesis implants have been around since the 1960s. Presently, the use of metal-on-metal implants has been increasing due to the fact that historically MOM systems have shown better than two orders of magnitude less wear than metal-on-polyethylene (MOP) systems. However, in the case of MOM devices, there is still concern regarding the long term effects of in vivo metal ion release due to corrosion and/or wear of the metal components.
The corrosion resistance of CoCrMo alloys is thought to be due to the existence of a natural metal oxide layer on the alloy surface. The major components of this oxide layer are cobalt oxide and chromium oxide. Because chromium oxide is much more resistant to leaching by body fluids than cobalt oxide, the corrosion resistance of the alloy depends on the continuity, thickness and chromium oxide content of the layer.
In the orthopedic industry, CoCr implants are routinely treated by immersion in 30% nitric acid solution at an elevated temperature (54° C.). It is believed by many in the industry that passivating CoCr materials with nitric or citric acid leads to the production of a corrosion resistant surface by forming a thin transparent Cr-oxide film. It is this layer that they believe imparts the corrosion resistance to CoCr implant materials. These beliefs are based on work published for Stainless Steel passivation. In reality, the surface of a nitric (citric) acid passivated surface has a surface composition of approximately 50:50 Co and Cr.
MOM implants are also subject to weight loss caused by the continuous friction of two contacted surfaces moving in body fluid. As these surfaces contact each other, the interaction of asperities or surface roughness causes wear.
To date, most efforts to reduce wear in MOM systems have focused primarily on reducing surface roughness and/or surface asperities. In particular, many investigators believe that removing surface “carbide asperities” will dramatically reduce break-in wear and yield a lower wear system. Many attempts have been made to remove surface carbides via enhanced polishing methods and heat treatment processes designed to “dissolve” the carbides into the alloy. These efforts, however, have produced little impact in terms of a significant reduction in break-in wear. Thus, there remains a need in the art to reduce break-in wear in prosthesis devices.